In ambulatory medicine, reusable injection apparatus or devices for subcutaneously administering liquid medicines are increasingly being used instead of disposable syringes.
One example of such reusable injection apparatus is injection pens which dispense a medicine in a desired amount from a pre-filled ampoule by means of a dosing means. Typically, for each injection, a new injection needle is placed on the injection pen and removed after it has been used. The needle is a hollow needle for conducting medicine from a storage container into the user's or patient's body.
Despite certain standards for injection apparatus, manufacturers prefer to develop injection apparatus which are only suitable specifically for their medicines. Thus, practically all such injection apparatus have a specific fixing device for the needle, which therefore does not fit or only insecurely fits onto another model or device. The needle is securely fixed on an injection pen when it neither slips nor tilts out of the perpendicular. An insecurely fixed needle increases the risk of injuries or needle breakage. If a physician or patient requires a number of medicines or injection apparatus for a treatment, they are furthermore reliant on a corresponding variety of needles, which makes the needles more difficult to obtain and store and, in smaller amounts, makes them more expensive.
Consequently, a fixing device has been sought for injection needles which securely and detachably fixes a needle on different models of injection apparatus, in particular on standard commercial injection pens having a thread or thread segments for detachably fixing or connecting the needle.
Fixing devices for injection needles, for pushing or screwing the needle onto the thread or thread segment, are known from documents WO 95/01812 and GB 737,676. A typical fixing device has the shape of a cap with one open end and one closed end which holds the needle perpendicularly in the middle. At least one elevation or recess, for example a cam, thread or thread segment, is situated on the inner side of the surface areas of the cap and snaps into the flight of the complementary thread or thread segment of the injection pen or is screwed in by being turned. The fixing devices of the above-cited patent specifications typically have cams which taper towards the thread with a pitch which is adapted to the thread, such that—when pushed onto a thread with a different pitch—the cap undesirably tilts out of the perpendicular if the cams engage with the flight.
To prevent tilting, at least three points of contacts are needed between the inner side of the cap or its cams and the thread of the injection pen, wherein said points of contacts are not in the same plane and furthermore hold the cap in the perpendicular. This means that, once pushed on, these points of contacts must no longer shift, otherwise the needle slips along the direction of the needle.
Although not provided for injection pens, U.S. Pat. Nos. 2,894,509 and 2,828,743 describe fixing devices comprising cams or grooves, which—independently of the pitch of the thread—are not in the same plane and enable two fixed positions and a defined shift of a needle. However, the solution from U.S. Pat. No. 2,828,743, with grooves which practically encompass the whole circumference, is not suitable for detachably pushing or screwing onto a thread. The solution from FIGS. 2 and 3 of U.S. Pat. No. 2,894,509 is only suitable for a thread or thread segments if the cams engage uniformly, secured against slipping, with the flights, which given the different dimensions of the threads is rarely the case.
In specific cases, the cap can also tilt out of the perpendicular, the upper or lower pair of cams describing a tilt axis. Consequently, more cams would be necessary to counteract this tilting symmetry. More cams furthermore provide the advantage that, through more points of contact, the clamping force in these points can be smaller for the same adherence of the cap or needle on the injection pen. The clamping force corresponds to a spring force which is predominantly determined by the geometry of the cams, the arrangement on the surface area of the cap and the elasticity of the materials. A smaller clamping force therefore allows, for the same spring force, a larger spring path of the cam or the surface area of the cap. Consequently, such a cap would fit onto correspondingly larger or smaller thread diameters, providing the clamping force always acts the same way, preferably perpendicularly, on the thread of the injection pen.
None of the known fixing devices for injection needles show a solution in which the spring forces in the points of contact always act the same way, in particular perpendicularly, on the thread, if the spring path is increased. This would be the case if the cams were attached in the middle of spring-elastic elements which were each connected to the rigid surface area via an end, analogously to a three-point bending element. Doubling the points of contact, in order to enable a larger spring path of the cams for equally larger differences in the diameters of the threads, required as least twice as much space on the inner side of the surface area of the cap. In the known solutions, with up to five cams at most, this is a very complicated design or cannot be solved at all, when the cams have to be arranged in different planes if the cap is not to tilt out of the perpendicular.